Hematite
Hematite, also spelled hæmatite, is the mineral form of Iron(III) oxide (Fe2O3), one of several iron oxides. Hematite crystallizes in the rhombohedral system, and it has the same crystal structure as ilmenite and as corundum. Hematite and ilmenite form a complete solid solution at temperatures above 950°C. Hematite is a mineral, colored black to steel or silver-gray, brown to reddish brown, or red. It is mined as the main ore of iron. Varieties include kidney ore, martite (pseudomorphs after magnetite), iron rose and specularite (specular hematite). While the forms of hematite vary, they all have a rust-red streak. Hematite is harder than pure iron, but much more brittle. Huge deposits of hematite are found in banded iron formations. Grey hematite is typically found in places where there has been standing water or mineral hot springs, such as those in Yellowstone. The mineral can precipitate out of water and collect in layers at the bottom of a lake, spring, or other standing water. Hematite can also occur without water, however, usually as the result of volcanic activity. Clay-sized hematite crystals can also occur as a secondary mineral formed by weathering processes in soil, and along with other iron oxides or oxyhydroxides such as goethite, is responsible for the red color of many tropical, ancient, or otherwise highly weathered soils. ("SEM"), magnification 20x]] Good specimens of hematite come from England, Mexico, Brazil, Australia and the Lake Superior region of the United States and Canada. Ochre The name hematite is derived from the Greek word for blood (haima) because hematite can be red, as in rouge, a powdered form of hematite. The color of hematite lends it well in use as a pigment. Ochre is a clay that is colored by varying amounts of hematite, varying between 20% and 70% . Red ochre contains unhydrated hematite, whereas yellow ochre contains hydrated hematite (Fe2O3 • H2O). The principal use of ochre is for tinting with a permanent color. Magnetism Hematite is an antiferromagnetic material below the Morin transition at 260 K, and a canted antiferromagnet or weakly ferromagnetic http://www.earthsci.unimelb.edu.au/ES304/MODULES/MAG/NOTES/typesmag.html above the Morin transition and below its Néel temperature at 948K, above which it is paramagnetic. The magnetic structure of a-hematite was the subject of considerable discussion and debate in the 1950s because it appeared to be ferromagnetic with a Curie temperature of around 1000 K, but with an extremely tiny moment (0.002mB). Adding to the surprise was a transition with a decrease in temperature at around 260 K to a phase with no net magnetic moment. Dzialoshinksi and later Moriya showed that the system is essentially antiferromagnetic but that the low symmetry of the cation sites allows spin–orbit coupling to cause canting of the moments when they are in the plane perpendicular to the c axis. The disappearance of the moment with a decrease in temperature at 260 K is caused by a change in the anisotropy which causes the moments to align along the c axis. In this configuration, spin canting does not reduce the energy. Hematite is part of a complex solid solution oxyhydroxide system having various degrees of water, hydroxyl group, and vacancy substitutions that affect the mineral's magnetic and crystal chemical properties. M.-Z. Dang, D.G. Rancourt, J.E. Dutrizac, G. Lamarche, and R. Provencher. Interplay of Surface Conditions, Particle Size, Stoichiometry, Cell Parameters, and Magnetism in Synthetic Hematite-like Materials. Hyperfine Interactions 117 (1998) 271-319. Two other end-members are referred to as protohematite and hydrohematite. Hematite on Mars partly embedded in rock at the Opportunity landing site. (Scale: image is approximately 5 cm (2 in) across)]] The spectral signature of hematite was seen on the planet Mars by the infrared spectrometer on the NASA Mars Global Surveyor ("MGS") and 2001 Mars Odyssey spacecraft in orbit around Mars NASA MGS TES Press Release, May 27 1998 "Mars Global Surveyor TES Instrument Identification of Hematite on Mars", available here. The mineral was seen in abundance at two sitesBandfield, J.L., Global mineral distributions on Mars, J. Geophys Res., 107, 2002. See: Mars Global Data Sets: Hematite Abundance. on the planet, the Terra Meridiani site, near the Martian equator at 0° longitude, and the second site Aram Chaos near the Valles Marineris Glotch, T. D., and P. R. Christensen (2005), "Geologic and mineralogic mapping of Aram Chaos: Evidence for a water-rich history," J. Geophys. Res., 110, E09006, doi:10.1029/2004JE002389 abstract here. Several other sites also showed hematite, e.g., Aureum Chaos T. D. Glotch, D. Rogers, and P. R. Christensen, A Newly Discovered Hematite-Rich Unit in Aureum Chaos: Comparison of Hematite and Associated Units With Those in Aram Chaos, Lunar and Planetary Science Conference XXXVI, 2005. Because terrestrial hematite is typically a mineral formed in aqueous environments, or by aqueous alteration, this detection was scientifically interesting enough that the second of the two Mars Exploration Rovers was targeted to a site in the Terra Meridiani region designated Meridiani Planum. In-situ investigations by the Opportunity rover showed a significant amount of hematite, much of it in the form of small spherules that were informally tagged by the science team "blueberries" (a term which is somewhat confusing, since in spectrally-correct color images they are, in fact, silver-grey in color). Analysis indicates that these spherules are apparently concretions formed from a water solution. Jewelry Hematite's popularity in jewelry was at its highest in Europe during the Victorian era, while in the last 50 years it has been popular in North America, especially in the western United States where it is found in jewelry and art created by Native Americans. Care should be taken in handling hematite items due to the material's susceptibility to damage. See also * Iron ore * List of minerals * Wüstite * Magnetite * Mineral redox buffer * Scientific information from the Mars Exploration Rover mission References *Martite - Mindat w/ location data *Iron rose - Mindat w/ locations External links * Abandoned Mine Research. * Mars spheres image. * Mars trench image showing a shiny texture of half-buried spheres (dark line is equipment shadow) * Florence Mine, Cumbria, UK. * MineralData.org Category:Oxide minerals Category:Iron minerals Category:Iron oxide pigments Category:Hematite group ca:Hematites cs:Hematit de:Hämatit et:Hematiit el:Αιματίτης es:Hematita eo:Hematito eu:Hematite fr:Hématite gl:Hematita hr:Hematit it:Ematite he:המטיט lt:Hematitas hu:Hematit nl:Hematiet ja:赤鉄鉱 no:Hematitt pl:Hematyt pt:Hematita ro:Hematit ru:Гематит simple:Hematite sk:Hematit fi:Hematiitti sv:Hematit uk:Гематит